An optical system of the type set forth at the outset is known from U.S. 2013/0 221 550 A1 and U.S. 2013/0 223 788 A1. DE 103 15 086 A1 has disclosed a method and an apparatus for aligning semiconductor wafers when producing semiconductors. The lithography apparatus disclosed therein includes an exposure unit, a first optical measuring device including an alignment microscope, and a further stray radiation measuring device. In this lithography apparatus, a wafer holder is arranged on a positioning device.